Patella prothesis have been used in total knee replacement surgery to insure a reproducible interaction of a patella with the femoral and tibial portions of the total knee replacement. Usually in such procedures the posterior side of the patella is prepared, sized and reamed so that the patella implant, when fixed to the patella, restores the reconstructed patella to its natural or original thickness.
Generally, during such procedure, the posterior side of the patella is exposed. The patella is then clamped so that the posterior side of the patella may be cut and then reamed. Prior to cutting and reaming the patella, a caliper is used to measure the preoperative patella height. In one procedure the patella is prepared for the patellar implant as follows. A patellar holding clamp is placed on the patella with a clamp ring on the posterior side. With the clamp on, a guide wire is inserted through the patella from a pre-marked location on the patellar posterior side through the patellar anterior side. The patella is planed with a cannulated planer through the clamp ring with the guide wire guiding the planer. A stop on the planer contacts the top of the clamp ring when a predetermined planing depth is reached. This depth is constant for each size. The patella is then reamed with a reamer inserted over the guide wire. The predetermined ream depth is determined when the reamer stop head having the same or smaller circumference as that of the cut plane, hits the planed bone. The patellar implant is then inserted into the prepared bed.
One disadvantage of this technique is the planing and reaming depth is referenced from where the clamp ring clamps the patellar bone. The system approximates the original patellar thickness based on an average thickness. Thus, although the system estimates original thickness, it does not provide adjustable cutting depths based on the variable of patellar dome height, i.e., the most prominent posterior point, typically the patellar median ridge.
In another patella implant procedure, after measuring the thickness of the patella, a depth stop plug is dropped down to contact the median ridge, i.e., the most prominent part of the patella anterior. The stop plug includes a ruler from which a measurement may be taken. The thickness of the patella and the measurement of the stop plug are used to calculate where to set the reamer stop. A reamer stop comprised of ring on the reamer is set to the scale so that the reamer will stop at the appropriate location. The final post operative height generally should correspond within plus or minus 1 mm of the original preoperative height, (unless another height is specifically desired). The reamer reams to a depth appropriate for the implant thickness. When the implant is in place, the restored height should be within 1 mm of the preoperative height.
One disadvantage of this technique is that it requires accurate measurements, e.g., of the thickness of the patellar bone and the location of the depth stop plug, and accurate calculations in order determine the depth of the ream for preparing the patella implant. Accordingly, one object of the present invention is to provide a patella referencing and reaming instrument which avoids the necessity to make specific measurements and calculations based on those measurements, in order to accurately size the reams and/or cuts in the patella.